This invention relates to fine powders, particularly fine powder comprising iron particles. In particular the invention is concerned with carbonyl iron which is rendered oxidatively resistant.
Powdered iron is used in a wide variety of applications. In powder metallurgy, such powder can be used to form a multitude of materials and shaped objects. The bonding of powdered particles into a mass of metal powder by molecular or atomic attraction into the solid state is effected by heating below the melting point of the metal. Sintering of the powder mass normally results in densification and often recrystalization.
Powders can be mixed in different forms and flowed into die cavities, and there formed into useful products under pressure or die molding. Heating is appropriately applied to obtain suitable product characteristics. Supplementary operations can be effected such as rolling or drawing thereby to obtain suitable machined products which can then be subjected to other finishing operations.
Resultant powdered metallic products have a metallic shape equivalent in function, although of lower density, and often of equivalent physical and mechanical properties to a wrought metal product. Such powdered metal products are produced faster, and normally at lower costs in terms of labor, material and energy.
A characteristic of different powdered metals is that the shape can vary from regular uniform spheroids to irregular spheroids, irregular spongy structures, dendritic, angular, flakey or leaf-like structures.
The particle size can vary from an average of about 2 microns to 80 microns. The method of fabricating the powder normally determines the size. Thus a carbonyl iron powder made by a carbonyl decomposition process produces fine particle size in a range of 1 to 20 microns, with a mean diameter of about 10 microns. An electrolytic process produces the average particle size of about 80 microns.
Different manufacturing processes produce different shapes. Thus the carbonyl process produces a substantially uniform spheroid particle shape, the atomization process produces round irregular spheroids and the electrolytic process produces a dendritic shape.
This invention is particularly concerned with carbonyl particles, namely those made by the decomposition of liquid or gaseous metal carbonyl (iron or nickel) to give a highly purified fine powder. This process is effected by applying heat to a composition of Fe(CO).sub.5 which then decomposes to iron particles and carbon monoxide.
When different powders are combined superalloys are obtained which can produce products with high melting points, composite metals, metal-non-metal combinations, porous metals, metals of extremely high purity, wear surface-coatings, and decorative-coatings such as gold or silver for use, for instance, in the graphic arts.
The multitude of applications of powdered metal parts often depends on the nature of the powdered metal. Components made from porous powdered metal include self-lubricating bearings, bushings and metallic filters and other structural entities and shapes. Products are for use with gas and liquids, and can be used for instance, in metering devices, distribution manifolds and storage reservoirs. Powdered metallic structures can be created by spraying metal onto a substrate.
Powdered metal tool steels include drills, knife blades, cutters, insert blades for gear cutters, and cutting and cutting tool inserts.
Powdered metal friction materials can be metal-non-metal combination. Such materials form clutch plates, brake pads and blocks and packing compositions. A sintered friction material can be composed of a metal matrix which includes copper and metal such as tin, zinc, lead and iron together with graphite and friction producing components such as silica or asbestos.
Powdered metallurgical electrical products constitute electrical contact elements such as tungsten contacts which are used for automotive and appliance applications. Often the use is limited because of an insulating oxide which forms during switching.
Copper and silver are combined with refractory materials such as tungsten, tungsten carbide and molybdenum in applications for power circuit-breakers and transformers, and tap-changers where they are confined to an oil bath because of the rapid oxidation in air. Where the contact is made of tungsten-silver, operation in air is possible because of the silver. Costs, however, are increased.
Powdered metal products also constitute permanent magnets and soft magnetic parts such as iron pole pieces for small DC motors and generators, cores for generators and radio transformers and measuring instruments. An iron powdered core for this purpose is coated with an electrically insulated material, compacted, ejected and baked to fuse the coated particles together. Such cores afford a large change of inductance by movement in one direction in or out of a wire wound-coil. Fine iron powder usually of electrolytic or carbonyl type is employed. Such cores exhibit minimum eddy current and hysteresis losses and the magnetic permeability returns to its original value after application of large magnetizing forces.
Other applications in this field include those of elements for incandescent lamps, electronic tubes and resistor elements. Refractory metals can be used to produce filament wire for incandescent lamps.
Additionally, powdered heavy metal compositions have important uses in electronics, alloying, nuclear power, chemical catalysts, metal cutting and forming, mining and drilling.
Cemented carbides containing tungsten carbide imbedded in a matrix of cobalt are used for parts requiring corrosion resistant. These include burnishing tools and dies, pump valves, nozzles, guages and drills.
High temperature applications are achieved with cermets which are metal ceramic combination. Cermets provide characteristics between cobalt/nickel base super alloys and refractory materials such as tungsten. Such mixtures have the high temperature strength of ceramics and sufficient ductility and thermocondutivity to provide resistance from thermo-shock at high temperatures and also workability at room temperatures. Composite materials can be formed with powder imbedded in elastomeric or ceramic binders.
One of the finishing treatments which can be applied to powdered metal products is plating. In general all types of plating processes, materials and products can be used, including copper, nickel, chromium, cadmium, and zinc. The plating is effected on a finished product. Entrapment of plating solutions in the pores of the product is avoided by sealing parts with resin impregnation.
Although plating of a finished structural product has been effected in the finishing process, it has not been applied to particles while in the powdered state. Indeed, as indicated, resin impregnation is employed to prevent the plating compositions and plating effects from penetrating the surface of the structural product.
Different plating processes are well known, and range from electroplating to electroless plating. The process of plating is the deposition of an adherent metallic coating on a substrate. Whereas electroplating requires an electric current, electroless plating uses an immersion process to effect the coating. By the plating process there is imparted to the substrate an improved corrosion resistance, appearance, frictional characteristic, wear resistance and hardness to the treated surface.
In engineering applications electroplates may be applied for improved mechanical, physical, and chemical properties. Nickel improves hardness, strength, and stress together with providing generally good resistance to corrosive chemicals.
The properties of the plating treatment vary according to whether electroplating or electroless plating is applied, and additionally the nature of the material which is being plating to the substrate.
The substrate being prepared for plating is usually cleaned mechanically and chemically and thereafter rinsed and possibly acid dipped. Depending on the material being plated and its intended use, different techniques of plating are applied and different metals can be plated onto the substrate. These would include nickel, copper, cobalt, gold and lead. Nickel and copper have particular advantages and are extensively used in a thickness from a mere flash to many millimeters. Alloy plating with nickel base materials is of particular interest in regard to magnetic properties, particularly in computer technology and where electroforming is required.
Electroless plating techniques and immersion procedures provide for deposits of limited thickness relative to electroplating techniques. This process achieves uniform plating at low capital cost since no DC power is required. Autocatalytic plating employs the deposition of a metallic coat by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited.
The more widely used electroless process is the electroless nickel process wherein nickel ions in solution are reduced to the metal by a reductant. The deposits usually provide good chemical and physical properties even though the initial cost may be high since a reducing agent such as sodium hypophosphite is required as well as precise control of the process.
In these cases usually a nickel/phosphorous alloy containing about 5% to 15% phosphorous is employed in a plating bath. Where electroless copper is used as a plating bath to provide products for the electrical industry, a reducing agent such as formaldehyde is used in a bath containing copper sulfate.
The applications of plated products include cases where oxidation protection and other special surface properties must be improved. Where the plating is for protection, for instance, of steel as a structural metal, paints and organic coatings containing zinc and cadmium electroplates which protect the steel substrate are widely used.
From the perspective of the plating industry also, while it has been common to plate a substrate of substantial mass it is unknown, and indeed has not been desirous to plate powdered metal particles themselves.
For many and various applications it has been found that a need exists to improve the resistance to oxidation of products employing powdered iron or being formed of powdered iron particles, particularly at higher temperatures. Despite the multitude of products and procedures which are available in the powdered metallurgy field and plating and extensive plating technology such a suitable oxidatively resistant product does not exist.
Other uses of powdered metal products are disclosed in the information disclosure statement filed contemporaneously with this application and incorporated by reference herein. None of the art disclosed there discloses powdered metal products having the appropriate oxidation resistance, particularly at elevated temperatures.